OSPF Route Filtering and Summarization

OSPF Route filtering between areas

  • Filtering prevents the creation of LSAs.
  • LSDB’s have to be the same for all routers ifnot SPF logic will fail
  • OSPF can filter the originiation of LSA between areas
  • Type 3 LSAs are filtered prior to origination ABR
      • With multiple ABRs filtering should be done on both.
      • ospf# area number prefix-list prefix name in|out
        • In: IOS filters routes comming in to that area
        • out: IOS filters routes comming out of that area
    ip prefix-list NAME seq 5 deny le 32
    ip prefix-list NAME seq 10 permit le 32
    R1#(config-router)#area 1 filter-list prefix NAME in
    R1#show ip ospf database summary self-originate

  • Type 5 LSAs are filtered at ASBR


  • Filtering with Area-Range
    • ospf# area number range <subnet><mask> not-advertise
    • Used for route summarization
    • not-advertise turns it into a filter
    • Doesn’t requite ACL or Prefix-Lists
    • Only works with Type 1 and Type 2 LSAs
R1#(config-router)#area 1 range not-advertise

OSPF Route filtering into the Route Table

  • Filter between OSPF database and Routing table.
  • Router still have LSAs for the routes but after running SPF these routes are filtered using a distribute-list and don’t make it in the routing table.
  • Can implemented on any OSPF router
  • distribute-list prefix NAME in <interface>
  • distribute-list ACL in <interface>
access-list 1 deny
access-list 1 permit any
R1#(config-router)#distribute-list 1 in
  • extended acl to deny a route from a specific neighbor
access-list 101 deny ip host
access-list 101 permit any any 
R1#(config-router)#distribute-list 101 in

OSPF Route Summerization

  • Only on ABR and ASBR
  • Creates new LSA
  • ASBR creates Type 5 LSA
R1#(config-router)#area 1 range


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